1. Field of the Invention
The present invention relates to a latent epoxy resin curing agent having excellent solvent resistance and low-temperature fast-curing ability and to a method for manufacturing the same.
2. Description of the Related Art
A latent epoxy resin curing agent having low-temperature fast-curing ability has been proposed in which a coating layer is formed on the surface of particles of an adduct of an imidazole compound and a polyfunctional epoxy compound through reaction with isocyanate (see Japanese Patent Application Laid-Open No. H05-331264). This latent curing agent is prepared as follows. The imidazole compound is added, in the presence of a dispersion stabilizer, to an organic solvent which can dissolve the imidazole compound and the epoxy compound but can not dissolve the adduct to be formed and which has a solubility parameter of 8 to 11. The mixture is heated to dissolve the imidazole compound. Then, the epoxy compound is added thereto and reacted with the imidazole compound to give a dispersion of the adduct particles, and the polyfunctional isocyanate compound is reacted with the adduct particles. The thus-obtained latent curing agent exhibits latency, i.e., does not initiate curing reaction at room temperature when mixed with an epoxy resin to be cured but initiates the curing reaction only after heated.
Meanwhile, it has been considered that an amphiphilic polymer compound having a high affinity both for the formed adduct and for the organic solvent can be preferably used as the dispersion stabilizer to be used upon preparing the latent epoxy resin curing agent described in Japanese Patent Application Laid-Open No. H05-331264 which contains the imidazole compound as a main component.
However, the dispersion-stabilizing ability of such an amphiphilic polymer compound is strongly affected, for example, by the chemical structures of the imidazole compound and epoxy compound and by the properties of the organic solvent. Therefore, when a person skilled in the art selects a suitable amphiphilic polymer compound, an excessive amount of trial and error is required. Moreover, in order to achieve sufficient low-temperature fast-curing ability, there is a strong demand for a latent epoxy resin curing agent that can provide an exothermic peak at 130° C. or less and an exothermic heat of 250 J/g or more in differential scanning calorimetry (DSC) of a thermosetting epoxy resin composition containing this curing agent.